The present invention relates to a position detecting apparatus for detecting the distance and position for and to which a movable component of a machine tool or industrial robot has moved.
A position detecting apparatus is known, which detects the distance and position for and to which a movable component of a machine tool or industrial robot has moved.
A position detecting apparatus of this type has been proposed in Japanese Patent Application No. 9-81016 filed by the present applicant.
FIG. 1 shows the position detecting apparatus proposed in Japanese Patent Application No. 9-81016.
As shown in FIG. 1, the position detecting apparatus 100 comprises a scale 101 and a magnetic sensor 102. The scale 101 is a magnetic member. The magnetic sensor 102 detects the magnetic field generated by the scale 101.
The scale 101 is a magnet that is made of, for example, ferrite-based plastic. It is shaped like a plate having a major surface 103. The major surface 103 is a length L11 and a width W10. The scale 101 has magnetized surfaces 103a and 103b. These surfaces 103a and 103b have been magnetized in opposite polarities, in a direction perpendicular to the major surface 103. As shown in FIG. 2, the magnetized surfaces 103a and 103b are separated by a boundary line m, which intersects with the longitudinal center line of the major surface 103, the center o of the major surface 103. (Hereinafter, the line m shall be referred to as xe2x80x9ccenter line n.xe2x80x9d The center line n and the boundary line m intersect with each other at an angle xcex8.
The magnetic sensor 102 comprises an annular core 104 and coils 105 and 106. The core 104 is shaped like a rectangular frame, defining a closed magnetic path. The coils 104 and 105 are wound around the opposing longer sides of the core 104, respectively. The core 104 is made of high-permeability material such as permalloy or amorphous metal. The coils 105 and 106 have 50 turns each. Each coil is formed by winding a Cu wire having a diameter of 0.06 mm around one longer side of the core 104. A high-frequency pulse current is made to flow in the coils 105 and 106, which generate magnetic fields in the opposite directions.
The scale 101 and the magnetic sensor 102, thus constructed, are spaced apart by a predetermined distance. They are arranged such that the coils 105 and 106 have their axes perpendicular to the major surface 103 of the scale 101 and set aside in the widthwise direction of the major surface 103 and that the midpoint between the axes of the coils 105 and 106 is located exactly above the center line n of the major surface 102.
The scale 101 and the magnetic sensor 102, thus arranged, are secured to a fixed component and movable component of a machine tool, respectively. The scale 101 and the sensor 102 may move along the center line n of the major surface 103 of the scale 101. At any position in the lengthwise direction of the scale 101, the magnetic sensor 102 detects a magnetic field emanating from the scale 101.
The operating principle of the position detecting apparatus 100 will be described, on the assumption that the lengthwise direction and widthwise direction of the major surface 103 are X direction and Y direction, respectively, and that the direction perpendicular to the major surface 103 is Z direction.
FIG. 3 shows a cross section of the scale 101, taken along a line passing the intersection o of the centerline n of the scale 101 and the boundary line in, or the center o of the major surface 103. As shown in FIG. 4, the magnetic field extending in the Z direction linearly changes in Y direction, from value xe2x88x92W10/4 to value W10/4. It follows that the magnetic field extending in the Z direction represents the position in the Y direction of the scale 101.
The scale 101 is designed so that its detection-effective length L12, measured lengthwise (in the X direction), is shorter than the length L11 (that is, L12 less than L11) and the angle xcex8, at which the lines n and m intersect with each other, is tan xe2x88x921(d/L12) (d is equal to or less than W10/2). The magnetic sensor 102 is moved along the center line n in the X direction, within the range of the detection-effective length L12. The two coils 105 and 106 respectively detect the magnetic fields that emanate from the surfaces 103 a and 103b magnetized in the opposite polarities and which extends in the Z direction. The difference between the outputs of the coils 105 and 106 linearly changes according to their positions along the longitudinal direction of the scale 101, in the same way as the magnetic field extending in the Z direction changes in the Y direction in the cross section of the scale 101, which is taken along a line passing the intersection o of the lines n and m. That is, the vertical magnetic field, which the magnetic sensor 102 detects, changes in accordance with the distance between the scale 101 and the magnetic sensor 102. This means that the positional relation between the scale 101 and the sensor 102 can be detected from the intensity of the vertical magnetic field the magnetic sensor 102 has detected.
The magnetic field generated by the scale 101 linearly changes, but for an extremely narrow range. The detection precision of the position detecting apparatus 100 described above is greatly influenced by a displacement, if any, of the scale 101 or the magnetic sensor 102. It is difficult for the apparatus 100 to detect the positions of the scale 101 and sensor 102 with a high accuracy. It is also difficult to position the scale 101 and the sensor 102 in the process of securing them to the components of a machine tool. Inevitably, the outputs of the coils 103a and 103b differ from the desired values. The detection precision of the apparatus 100 greatly decreases when the magnetic sensor 102 is positioned outside the region in which a magnetic field changes linearly.
The magnetic sensor 102 is a coil sensor. Its dimension is the largest in the coil-winding direction, i.e., magnetism-detecting direction. In the position detecting apparatus 100, the magnetic sensor 102 detects the magnetic field that extends in the direction perpendicular to the major surface 103 of the scale 101. The magnetic sensor 102 must therefore be arranged so that the magnetism-detecting direction may be perpendicular to the major surface 103 of the scale 101. Consequently, the apparatus 100 is large and massive.
The present invention has been made in consideration of the foregoing. The object of the invention is to provide a position detecting apparatus that is small and exhibits output characteristic little influenced by a positioning error, vibration, or displacement.
To achieve the object, a position detecting apparatus according to this invention comprises: magnetic field generating means for generating a magnetic field; and magnetic field detecting means capable of moving relative to the magnetic field detecting means, for detecting the magnetic field generated by the magnetic field detecting means. The magnetic field extends at right angles to a direction in which the magnetic field detecting means moves relative to the magnetic field generating means and extends to a magnetism-sensing direction in which the magnetic field detecting means is spaced from the magnetic field generating means. The magnetic field generating means applies to the magnetic field detecting means a magnetic field whose intensity linearly changes over a prescribed distance (L2) in the direction in which the magnetic field detecting means moves relative to the magnetic field generating means. The positional relation between the magnetic field generating means and the magnetic field detecting means is detected from the magnetic field detected by the magnetic field detecting means.
In the position detecting apparatus, the magnetic field generated by the magnetic field generating means extends at right angles to the direction in which the magnetic field detecting means moves relative to the magnetic field generating means. The intensity of this magnetic field linearly changes in the direction in which the magnetic field detecting means moves relative to the magnetic field generating means. The magnetic field detecting means detects the magnetic field that changes in intensity in accordance with the positional relation between the magnetic field generating means and the magnetic field detecting means. The positional relation is determined from the magnetic field detected by the magnetic field detecting means.
As mentioned above, the magnetic field generating means generates a magnetic field that extends at right angles to the direction in which the magnetic field detecting means moves relative to the magnetic field generating means. The intensity of the magnetic field linearly changes in the direction in which the magnetic field detecting means moves relative to the magnetic field generating means. The magnetic field detecting means detects this magnetic field, from which the positional relation is determined.
The position detecting apparatus according to the invention has output characteristic that is little influenced by a positioning error, vibration, or displacement. Moreover, the position detecting apparatus can be small.